Earth Science: Biology – Plants and Fungi – Water and Nutrient Transport
Unlike animals, plants occupy a fixed location. In order to successfully grow and reproduce, they must adapt to local light, temperature, water, and soil conditions.
In their green leaves, plants produce organic nutrients through photosynthesis, while the roots take up water and minerals from the soil. All of these substances are used by the whole plant.
To distribute them effectively, plant tissues are permeated by transport channels: the vascular bundles.
Xylem
In addition to water, plants need many inorganic substances for metabolism and growth, such as potassium, calcium, phosphate, magnesium, and nitrogen compounds. In higher plants, these sub- stances are absorbed through the root hairs and transported through an internal distribution system known as the xylem.
Xylem tissue includes the actual transport channels, in the form of long hollow tubes (xylem vessels) whose living protoplasm has died off. The xylem system also contains fibrous cells and storage tissues that help reinforce and strengthen the plant. This distribution system uses a passive, energy-efficient mechanism.
Leaves and phloem
In the leaves, transpiration takes place through the pores, as water vapor evaporates into the less-humid sur-rounding air. This produces a suction force that draws liquid from the plant’s cell walls. Together with the xylem vessels they form a closed capillary system,
which constantly draws water and minerals up from the roots.
In addition to the xylem, a plant has another transport system. It distributes organic compounds, such as carbo-hydrates and amino acids. These substances, produced in the plant’s green tissues, must be carried to other parts of the plant. This long-distance transport takes place in tissue known as phloem.
Especially high concentrations of sugars can be found in the phloem vessels, which are called sieve tubes. In the leaves, sugars are actively transported into the phloem, later being removed in the areas where they are needed.
PHLOEM RESEARCH
Aphids feed on the concentrated sugar solutions in the phloem by inserting their sharp mouthpieces into sieve tubes. Biologists use this
ability of aphids to study the content of the phloem.
Using a laser, they detach the aphid’s mouthpiece, collecting the phloem liquid in tiny tubes. They also analyze the honeydew fluid emitted by aphids. Scientists have discovered that the phloem carries messenger substances, hormones, viruses, and nucleic acids.
BASICS
MANY PLANTS have symbiotic relationships with soil fungi that enhance the plant’s ability to take up water and minerals, while receiving nutrients from the plants
IN TREES and other woody plants, the actual wood is formed from xylem tissue, which has undergone a secondary growth process.